1. Field of the Invention
The present invention relates in general to developing a spray dryer/bag filter apparatus for removing pollutants contained in flue gas from an incinerator and, more particularly, to use a part of treated flue gas as the cooling shield air for spraying part of spray dryer. Also, the present invention is concerned with a method for removing pollutants contained in flue gas from an incinerator using the apparatus.
2. Description of the Prior Art
A Flue from a municipal waste incinerator contains typically various environmental pollutants, such as fly ash, acid gases (HCl, SO.sub.2, HF, H.sub.2 S), heavy metals (Hg, As, Pb), dioxins (PCDDs, PCDFs), and volatile organic compounds (VOC). Among these, acid gases, including hydrogen chloride (HCl), sulfur dioxide(SO.sub.2), hydrogen fluoride (HF), and hydrogen sulfide (H.sub.2 S) are removed in spray dryer while fly ash and dried spray particle are filtered off by bag filter. The bag filter can also remove heavy metals and dioxins absorbed in the fly ash and dried spray particles.
For the better understanding about the background of the present invention, a conventional technology will be described in FIG. 1.
FIG. 1 shows a full procedure of flue gas treatment with a conventional apparatus comprising a spray tower, spray dryer, bag filter and SCR(Selective Catalytic Reduction) reactor.
Flue gas from an incinerator(1) proceeds to a spray tower in which temperature and humidity of the flue gas are controlled to the optimal condition for the subsequent process. Then, the flue gas also proceeds to a spray dryer(3) at the upper or lower part of which provides slaked hydrated lime(Ca(OH).sub.2) slurry. The lower or upper part of the spray dryer is equipped with a spray nozzle through which absorbent fine particles are reacted with contaminant gas. With the reaction in the spray dryer(3), the exhaust acid gases are neutralized and the product is dried therein. These processes allow pollutants of the flue gas to be removed in the spray dryer(3). In the next stage, the scattered particles of the reaction product are moved into a bag filter(4) and filtered therein. Because the dry product remains unreacted and fly ashes are gathered at the surface of the bag filter(4), forming a layer, the acid gases which was not removed in the previous processes can also be removed with the additional reaction.
Also, this process can eliminate considerable amounts of dioxins(PCDDs, PCDFs) and heavy metals absorbed in the fly ashes. The nitrogen oxides (NO, NO.sub.2) and dioxins(PCDDs, PCDFs) still remain unfiltered are moved into a catalyst reactor(7) through an ID-fan(5) and Heat-exchanger(6). Just before entering the SCR reactor(7), the residual pollutants are mixed with ammonia, which is sprayed from an ammonia tank(13), and subjected to reduction and decomposition reaction on the surface of a catalyst in the reactor(7). After that the purified gas is drained out through a stack.
Spraying part(ref. FIGS. 1 and 3) of the spray dryer(3) is composed of an atomizing nozzle(24), high pressure air conduit(16), hydrated lime slurry conduit(15), and shield air conduit(17). The atomizing nozzle is connected with a slurry storage tank, a high pressure air conduit (16) and a shield air conduit (17). In the slurry storage tank (12), slaked lime slurry is prepared by mixing hydrated lime and water. In the high pressure air conduit(16), high pressure air is provided from a compressor(9). The cooling shield air is provided with an external air by a blower (14).
As a core equipment, the spraying part serves the spray dryer with the slaked lime slurry to fine particles 70-100 .mu.m in diameter in order to expand the contact area between the pollutants in the flue gas and the slaked lime slurry to a degree enough to facilitate the neutralization.
In the slurry conduit (15), the slaked lime is in a slurry state of 7-10% suspension. This slurry may be accumulated in the bottom of the conduit(15) and finally clog up the conduit, which requires a periodical inspection in order not to cause to hinder the operation of the apparatus. Further, after operation, the accumulated slurry must be completely eliminated from the conduit by water or air etc.
The high pressure air introduced by the air conduit (16) plays an important role in determining the particle size of the slaked lime slurry sprayed from the spray nozzle. Usually, the air has a pressure of 3-7 kg/cm.sup.2. If the diameter of the slurry sprayed is below 70 .mu.m, the slurry particles are prematurely dried, so that the removal efficiency of contaminant gas is significantly decreased. On the other hand, if the slurry particles are above 100 .mu.m in diameter, they are dried so slowly that they cause the particle accumulated on the wall surface of the spray dryer(3), causing a trouble in operating the apparatus. The shield air is provided with cooling air through the conduit (17) with the aim of preventing the direct contact of the spray nozzle with the hot flue gas.
As described above, the removing apparatus of the flue gas from an incinerator, comprising the spray dryer(3), the compressor(9), the slurry tank (12), the blower (14) and the bag filter(4), is called spray dryer/bag filter apparatus, and the process carried out therein is called spray dryer/bag filter process.
In the aforementioned conventional spray dryer/bag filter apparatus, the shield air to be provided to the spraying part of the spray dryer is taken from the exterior by the blower (14). The shield air is the largest amount of introduced external air into the process and make the flue gas dilute in the conventional process. Such an air introduction increase an oxygen contents of the flue gas to be treated. Consequently it decreases the removal efficiency of contaminant from the spray dryer (3). Further, the external air has a serious influence on controlling the optimum oxygen contents necessary to remove other pollutants in the next catalyst reactor (7). For example, in consideration that the optimal oxygen contents for removing NO.sub.x and dioxin is 7-9%.